(1) Field of the Invention
The present invention relates to a process for an electroless plating of copper for forming a copper film, for example, of a conductor circuit on a printed circuit board or a ceramic substrate, and a plating solution for use in carrying out this plating process.
(2) Description of the Related Art
As the electroless copper plating solution for chemically depositing metallic copper, there have ordinarily been used solutions containing ethylenediamine-tetraacetic acid (EDTA) or Rochelle salt as a complexing agent for a copper ion, copper sulfate as a copper salt, formalin or formaldehyde as a reducing agent and caustic soda as a pH-adjusting agent. However, these electroless copper plating solutions do not sufficiently spread because the copper-deposition speed is extremely low, several .mu.m per hour, and the physical properties are insufficient.
Under this background, the present inventors previously developed and disclosed an electroless copper plating solution providing a copper film having excellent physical properties at a high deposition speed (at least 30 .mu.m/hr, especially at least 100 .mu.m/hr) by using a trialkanolamine, which is a highly stable complexing agent for a copper complex, as a complexing agent and accelerator (U.S. Pat. Application Ser. No. 117,079 filed on Nov. 5, 1987now U.S. Pat. No. 4,834,796.
However, it was found that if plating is carried out by using this electroless copper plating solution, a new problem arises in that a uniform copper layer is not formed or a copper layer is not formed at all because of the effects of a catalytic activity at the surface of a substrate.
The present inventors investigated the causes of this problem, with the following facts found.
Namely, an electroless copper plating solution containing, as a complexing agent, triethanolamine having a high copper ion complex stability constant (also called "complex-forming constant") reacts selectively with a portion of the surface of a substratehaving a high catalytic activity. Accordingly, if a palladium layer is formed as s catalyst on the surface of a resin substrate, since it is generally difficult to form the palladium layer uniformly on the surface of the substrate and a large dispersion of the catalytic activity is produced on the surface of the substrate, it is impossible to form a uniform copper layer on the surface of the substrate.
On the other hand, where it is intended to perform an electroless plating of copper on a molybdenum or tungsten layer formed as a conductor pattern on the surface of a ceramic substrate such as alumina, since the catalytic activity of molybdenum or tungsten is much lower than that of palladium, it is impossible to deposit a copper layer by using the above-mentioned electroless copper plating solution containing triethanolamine.
As apparent from the foregoing description, in case of an electroless copper plating solution containing a complexing agent having a high stability constant as a copper complex, such as triethanolamine, a uniform copper layer is not formed or a copper layer is not formed at all because of the effects of the catalytic activity at the surface of a substrate due to a high catalytic selectivity of the plating solution.
It is therefore a primary object of the present invention to provide an electroless plating process in which a uniform copper layer can be formed even when a dispersion of the catalytic activity on the surface of a substrate ocurrs, and a copper layer can be formed even if the catalytic activity at the surface of a substrate is low.
Furthermore, where plating is carried out by using the above-mentioned electroless copper plating solution proposed by the present inventors, if maintenance of the plating solution is not properly carried out, the problem a stopping of the plating reaction arises, entirely or partially on the substrate surface before a desired plating thickness is obtained. The same problems arises when the above-mentioned conventional electroless copper plating solutions are used.
The cause of the stopping of the plating reaction has not been fully clarified. The phenomenon commonly observed in all plating solutions is that if the plating reaction is stopped, a passive state film is formed and even if this film is removed by an acid treatment to expose a fresh plated surface (Cu surface), reaction can rarely be restarted. Namely, it is considered that the generation of this passive state is not due to a deterioration of the plating solution but to a reduction of the catalytic activity of the plated surface.
As means for eliminating this trouble, there are known two methods; that is, a method in which a generation of the passive state is prevented by reducing the adsorbing capacity of the additive (Japanese Unexamined Patent Publication No. 60-70183) and a method in which plating is first conducted for a short time, a catalytic metal having a high activity, such as Pd, is adsorbed again and plating is then conducted for a long time (Japanese Unexamined Patent Publication No. 57-192099). However, use of an additive having a reduced adsorbing capacity according to the former method will naturally have an adverse effect on the plating characteristics, and in the latter method in which Pd or the like is adsorbed midway in the plating operation, it is apprehended that the Pd will adhere to a portion where plating is not desired as a product, and a problem of a formation of a short circuit in a formed circuit will arise.
Therefore, another object of the present invention is to provide an electroless copper plating process in which even if the catalytic activity is reduced by an improper maintenance of the plating solution or the like, electroless copper plating can be stably performed on the surface to be plated at a high speed.